I annotate screenshots every day. In iOS 11 there is a new markup tool for this. As soon as you have made a Screenshot there is a thumbnail of it in the lower left corner. Tap on it and you will be taken to the markup tool. Among the tools you find pencil, pen, marker, loupe, signatures and arrows. You can also crop the Screenshot or photo by dragging its edges. This is a huge productivity boost for all those who doesn't have very advanced requirements for markup tools. It will also be available in many apps, like Mail, Safari and Photos.